We plan to continue our efforts to elucidate the mechanism whereby the antibiotic pactamycin interferes with the initiation of polypeptide chains in cells from eukaryotes. This will involve studies on the binding of radioactive pactamycin to ribosomes at various stages during protein synthesis and study of the specific step inhibited that leads to the accumulation of inactive initiation complexes in fractionated systems and of initiator dipeptide in reticulocyte lysates. We have also begun to study the mechanisms whereby the antitumor antibiotic neocarcinostatin interferes with DNA replication in HeLa cells, and leads to DNA strand scission. Bibliographic references: Goldberg, I.H.: Kanchanomycin. In Antibiotics III. Mechanism of Action. D. Gottlieb and P.D. Shaw, Eds. Springer-Verlag, New York, 1975, pp. 166; Goldberg, I.H.: Pactamycin. In Antibiotics III. Mechanism of Action. D. Gottlieb and P.D. Shaw, Eds. Springer-Verlag, New York, 1975, pp. 498.